1. Field of the Invention
The present invention relates generally to frequency multiplier circuits, and more specifically to frequency doubler circuits for producing a relatively pure output for use in radio equipment.
2. Discussion of the Related Art
In radio communications, in order to reduce the number of oscillators, it is desirable to multiply the frequency of a local oscillator to effectively derive another local oscillator. For example, a frequency doubler is a circuit that produces an output signal at the twice the frequency of the input signal, the input signal or fundamental frequency being suppressed from the output. Typically, frequency doubling is realized by feeding the input signal through nonlinear devices, such as diodes or transistors, and then extracting the doubled or first output harmonic and rejecting the other frequency components.
One common frequency doubler is a rectifier type doubler that rectifies and doubles an input signal, then uses complex LC filtering to remove the unwanted harmonics. However, it is very complicated to implement such filtering on an integrated circuit and to remove the unwanted harmonics and maintain low noise over a wide input signal level range, such as from −10 to 0 dBm. Additionally, due to poor inductor Q (quality factor) and grounding in an integrated circuit implementation, it is very difficult to achieve good filter response for proper harmonic and spurious rejection.
Another approach is an injection lock type doubler, which is known to be very unreliable, e.g., gets out of lock easily. Such injection lock type doublers also require complex filtering to reject unwanted harmonics and spurious content.
Another well known approach is an unbalanced multi-tanh doubler, which is an ideal frequency doubler that generates only the desired doubled harmonic. The unbalanced multi-tanh doubler includes unbalanced emitter-coupled differential pairs with emitter area ratio K and relies on current summation and cancellation. Again, complex filtering is required for the rejection of unwanted harmonics. Disadvantageously, this doubler has a limited dynamic range with acceptable noise output performance. Thus, it is not able to maintain a low distortion output for a wide range of input levels and amplitude fluctuations.
It is with respect to these and other background information factors that the present invention has evolved.